The recent increases in UV-radiation reaching the biosphere have heightened our interest in understanding the mechanisms by which organisms protect themselves against this environmental stress. The goal of the propose research is to use a multicellular higher eukaryote as a model system to increase our understanding of the mechanisms that result in increased tolerance of UV-radiation. special attention is given to the molecular and cellular basis of UV-tolerance in plants since they are inevitably exposed to UVB radiation. The dicot plant Arabidopsis thaliana has been chosen because of its versatility as a model for studies in both classical an molecular genetics. I propose a set of approaches using diverse tools (genetic, biochemical, molecular an physiological) to learn more about the role specific mechanisms play in the overall tolerance to UV. In our laboratory we have defined a set of Arabidopsis ecotypes that display large variations in UV-resistance. One of the proposed approaches is to first define and then study the specific protective mechanisms (e.g. flavonoid accumulation, protection against free radicals, DNA repair, etc.) which are involved in this natural variation. We have also isolated a chemically generated mutant which shows a severe sensitivity to UV-light because of deficiency in flavonoid accumulation. This mutant is becoming a valuable tool to further understand UV-tolerance in plants. The second approach proposed makes use of the effect the lack of certain flavonoids has on UV tolerance in this mutant to thoroughly analyze the specific role played by flavonoids in UV- resistance (e.g. is only a UV-screening mechanism?). The third approach makes use of the severe UV-sensitivity of this mutant (and of certain UV-sensitive ecotypes) to search genetically for other protective mechanisms that may be able to compensate for the lack of flavonoids and increase UV-resistance. Completion of this project has thus, the potential of elucidating various mechanisms of UV-tolerance and their contribution to protection against UV in plants. At the same time, this research is likely to provide useful knowledge and new avenues of research towards a better understanding of the molecular mechanisms of tolerance to environmental stress. Finally, it is expected that various molecular and genetic tools as well as novel techniques will be generated which will be useful for similar studies in other organisms.